Many types of optical and electro-optical elements require the deposition of at least one, but typically multiple patterned thin film coatings onto a substrate. In order to obtain the optimum performance from these optical and electro-optical elements and to achieve satisfactory fabrication yields, it is necessary to provide precise control over the materials deposition process. One key aspect of this control relates to monitoring and controlling the thickness and materials composition of each thin-film layer. Because of the critical dimensions involved, it is advantageous to be able to accurately measure thin film thickness and composition not only once the layer has been deposited, but also in situ, as the material is being deposited. That is, it is desirable to be able to measure layer thickness and rate of change of layer thickness as well as to determine the relative composition of layer component materials dynamically.
There are inherent difficulties that complicate the measurement process in the thin film deposition environment and that make some conventional approaches unworkable for in situ measurement. Chief among these problems is the materials deposition process itself. For many thin-film deposition techniques, any component in the deposition chamber will be coated to some degree. Thus, the performance of optical components and sensors will degrade over time due to deposition of material onto these control components.
This problem has been exhibited with so-called crystal mass-sensor devices, used in many thin-film development environments. In the crystal mass-sensor device, the monitor is a quartz crystal having two opposing electrodes. The crystal itself is part of an oscillator circuit provided in a deposition rate monitor. Within an acceptable range, a frequency of oscillation of the oscillator circuit is approximately inversely proportional to a mass loading on a surface of the crystal occasioned by a layer or by multiple layers of material deposited on the crystal. This works acceptably, for a time. However, when the acceptable range of mass-loading of the crystal is exceeded, for example by build-up of an excess number of deposited layers, the oscillator circuit can no longer function reliably, necessitating replacement of the “overloaded” crystal with a new crystal mass-sensor. Such replacement, in turn, requires discontinuation of the vapor deposition process. In addition, when certain organic layers are deposited onto crystal mass-sensor devices there can be a tendency for the layers to start cracking and flaking from the mass sensor surface after coating thickness build-up on the order of 500-2,000 nanometer (nm). This can cause the crystal mass-sensor to become inaccurate in its coating rate measurement capability at thicknesses well below the aforementioned mass-loading limit. Thus, although the crystal mass-sensor device provides an acceptable solution for prototype and development work, this type of device, deteriorating with use and requiring regular replacement, would not be well suited for mass fabrication environments.
Similarly, deposition of material onto control and sensing components also has an impact on solutions that employ optical methods for in situ thickness sensing. Lenses, photosensors, or other optical components that are exposed within the deposition chamber are all subject to this problem. For this reason, a number of solutions propose the use of a surrogate “witness plate” that can be subjected to the deposition process and removed after a period in order to allow accurate layer measurement outside the deposition chamber. However, such a solution requires space in the deposition environment, requires an interface for its removal and reinsertion, introduces additional surface area and waste, and necessitates time delay so that the ability to obtain dynamic measurement data is compromised.
Among optical solutions for measurement that have been proposed is in situ fluorescence. For example, commonly assigned U.S. Pat. No. 6,513,451 entitled “Controlling The Thickness Of An Organic Layer In An Organic Light-Emitting Device” to Van Slyke et al. discloses a thin-film measurement method using a rotating disk member whose rotation exposes it to the deposition environment and also to monitoring and disk cleaning apparatus mounted just outside the chamber. Although this method resolves a number of difficulties, however, it requires that the area to be measured be a surrogate area rather than the device being formed and that this measured area be moved out of the coating process to another position to be measured. Although fluorescence may provide a suitable mechanism for measurement with some types of coatings, this method may have limited uses with other types of thin-film materials.
In another measurement method, U.S. Pat. No. 6,646,753 B2 entitled “In-Situ Thickness And Refractive Index Monitoring And Control System For Thin Film Deposition” to Zhang et al. discloses a transmission type measurement in which laser light sources at 2 distinct wavelengths are measured, with blocked beam and unattenuated. These measurements are used to obtain data on either index of refraction or thickness of the coating, or both. Optical measurements are made outside of the deposition area, using viewports in a vacuum deposition chamber.
U.S. Patent Publication No. 2004/0239953 entitled “Optical Method Measuring Thin Film Growth” to Flynn describes a method of measuring the rate of change of optical thickness of a thin-film during deposition in transmission by looking for the change in location of the transmission maximum wavelength caused by interference effects as a function of coating thickness. This approach is then used to sense the deposition rate by tracking the peaks in the transmission spectrum as a function of time.
US Patent Publication 2004/0008435 A1 entitled “Optical Film Thickness Controlling Method, Optical Film Thickness Controlling Apparatus, Dielectric Multilayer Film Manufacturing Apparatus, and Dielectric Multilayer Film Manufactured Using the Same Controlling Apparatus or Manufacturing Apparatus” by Takahashi et al. describes a method of determining layer thickness for dielectric layers from a transmission measurement using monochromatic light directed through a chamber window, and applying a calculation involving the reciprocal of the transmittance as a function of coating thickness.
Japanese application JP2004134154A entitled “Organic EL Device and its Production Method” assigned to Sanyo Electric Co Ltd., to S. Masakuzu, T. Teiji, and I. Hiroaki, describes a method for measurement and control of dopant concentration in a host material layer by measuring the fluorescent spectrum or light absorption spectrum of the layer. The '4154 Masakuzu et al. application also describes a control loop using in situ feedback on relative concentrations, allowing coating process adjustment. Dopant concentration is determined from the fluorescence spectrum by measuring at the wavelength that shows the maximum fluorescence intensity. The light absorbance is then used to determine the host concentration. The light source may be on either side of the substrate. However, there is no acknowledgement of the problem of keeping the light source clean from contamination in this in situ arrangement or of maintaining equal power throughout the process.
U.S. Patent Application No. 2004/0131300 by Atanasov entitled “Optical Monitoring of Thin Film Deposition” describes an optical monitoring system for monitoring thin film deposition on a substrate comprising a bridge supporting a pair of facing fiber optic collimators for a transmission measurement through a substrate.
U.S. Patent Application 2005/0046850 A1 entitled “Film Mapping System” by Chow describes a material's property measuring system for monitoring the reflection and transmission of electromagnetic radiation from a sample using a complex optical system with beamsplitters that spread light over the sample surface and using a detector array.
Although the methods described in the above listing may provide some measure of accuracy in determining layer thickness, there is a significant need for improvement. For example, approaches such as those outlined in the Chow '6850 application and '1300 Atanasov application are not suited to in situ measurement. In situ measurement would provide the most highly accurate data for determining the rate of change of deposition, useful in maintaining precision control of the deposition process. The '4154 Masakuzu et al. application does not address the problem of contamination from deposited material. More significantly, the methods described in the above-cited patent literature may perform adequately for single layer deposition, but do not work as well in measurement for components having multiple overlaid patterned layers.